1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly to an organic electroluminescent device solving low external quantum efficiency problems of surface plasmon resonance.
2. Description of the Related Art
Organic electroluminescent devices are also known as organic light emitting diodes (OLED). The OLED luminescent principle applies a voltage to organic molecular material or polymer material, and the device luminesces. Due to OLED's self emission characteristics, it can form a dot matrix type display with light weight, slim profile, high contrast, low power consumption, high resolution, fast response time, no need for backlighting, and full viewing angle. Possible display parameters range from 4 mm microdisplay to 100 inch outdoor billboards, making it a preferred type of flat panel display (FPD). If the OLED luminescent efficiency is over 100 Lm/W, it can replace conventional lighting.
In organic electroluminescence, electrons are propelled from a cathode layer and holes from an anode layer, and the applied electric field induces a potential difference, such that the electrons and holes move and centralize in a thin film layer, resulting in recombination. The energy of this recombination excites the luminescent layer moleculars to higher energy levels and unstable excited states, and when the energy is released, they return to lower energy levels and stable ground states. OLED luminescent efficiency depends on the internal and external quantum efficiency of the device. Internal quantum efficiency is the internal efficiency of converting electricity to light. After exciting the organic moleculars, a quarter of the excited electrons assume a single-state asymmetric spin configuration, releasing energy in the form of fluorescence. The other three-quarters assume triple-state symmetric spin configuration, and release energy in the form of phosphorescence. The triple state excited electrons also release energy in the form of phosphorescence in organometallic compounds. Therefore, OLED internal quantum efficiency depends on the excitation mechanism, and on the fluorescence or phosphorescence of luminescent material chosen.
OLED external quantum efficiency is the ratio of luminescent output from device to the luminescent from the organic layer. In a typical OLED, not all light from the organic layer can pass through the device, with more than 40% of OLED light lost to surface plasmon resonance. In addition, the organic material and the glass substrate have a higher refraction index than air, so some light is limited in the device due to total reflection, some scattering outward from the device side. Around 80% of light is dissipated in the device, making conventional OLED external quantum efficiency below 20%. In the unused device light can be recovered, the OLED external quantum efficiency improves.